In response to the increased cost and diminishing quality and availability of natural wood, composites consisting essentially of plastic and natural fibers are steadily replacing the use of traditional wood in construction and transportation applications. Unlike natural wood that splinters and warps, the composites are weather resistant and relatively maintenance free but still offer the same look and feel as natural wood.
The composites typically comprise a plurality of fibers imbedded in a polymeric material. The polymeric material typically consists of a high or low-density olefin thermoplastic or a vinyl based thermoplastic polymer depending on the desired end-use characteristics of the composite. The fibers may be chosen from a variety of plants depending on the desired characteristics of the fiber, for example, density or strength. The natural variation in the apparent density of the different plant fibers is attributable to the presence of a central void or lumen within the fiber.
The manufacture of the composite typically involves extruding of the polymeric material and the fiber. During the manufacture thereof, an extruder melts the polymeric material and mixes the melted polymeric material with the fiber. As a result of the mixing, the melted polymeric material becomes imbedded with the fiber. A bonding agent may be added to the mixture to aid in achieving an adhesive bond between the fiber and the polymeric material. Many other “additives” may be introduced, such as, stabilizers, antioxidants, UV absorbers, fillers and extenders, pigments, process aids and lubricants, impact modifiers, bactericides and other materials that enhance physical and/or chemical properties as well as processing. A chemical blowing agent or gas may also be introduced into the mixture. While in the extruder, the blowing agent decomposes, disbursing a gas, such as, nitrogen or carbon dioxide, into the melted polymeric material. After the polymeric material, fiber and other additives are mixed, the melted mixture exits the extruder through a die. As the polymeric material exits the die, the extrusion pressure is reduced to atmosphere and the polymeric material begins to cool causing the entrained gases to expand as bubbles within the melted mixture. The bubbles are trapped by the surrounding polymeric material and form voids in the composite. These voids reduce the overall density and weight of the composite.
Often during extrusion, the lumen in the fiber collapses under compressive pressure. When the lumen collapses the natural voids in the fiber are lost causing the natural density of the fiber to increase. Because the density of the fiber is increased, the mass of the composite also increases. This increased density runs counter to the advantages of using fiber, which is mass reduction and stiffness enhancement.
It is therefore desirable to develop a fiber and a method of manufacture thereof wherein the lumen does not compress during extrusion such that the natural voids of the lumen are preserved causing the fiber to maintain natural density and strength characteristics. It is also desirable to reduce the overall composite mass by using a blowing agent to further introduce void volume within the polymeric material.